The present invention relates to a cold-cathode fluorescent lamp used for backlighting of a liquid crystal display and the like.
A cold-cathode fluorescent lamp used as a light source for backlighting of a liquid crystal display is configured such that cylindrical or plate metal is provided as an electrode in a lighting tube, which has phosphor applied onto the inner surface of a glass tube, mercury and the like is contained therein, and the phosphor is excited by ultraviolet radiation, which is generated by electric discharge in the lighting tube, to provide visible radiation.
As to such a cold-cathode fluorescent lamp, a variety of studies on miniaturization, a smaller diameter, higher intensity, and longer lifetime have been conducted in response to diversity of liquid crystal displays. For example, Japanese Patent Laid-Open No. 1-151148 proposes a cold-cathode fluorescent lamp, in which in order to reduce consumption of mercury in a lamp in high-power electric discharge and to optimize a discharging area of an electrode, a cylindrical electrode made of metal is provided on an end of a lighting tube to obtain longer lifetime.
However, in the cold-cathode fluorescent lamp configured thus, when the lamp has a relatively large current of 5 mA or more and the lighting tube has an extremely small inside diameter of 1 to 6 mm, both of the inner surface and outer surface of the cylindrical electrode are subjected to electric discharge. Thus, electrode sputtering materials generated by electric discharge are increased, thereby accelerating a so-called mercury trapping phenomenon, which consumes mercury in the lamp. Consequently, longer lifetime of the cold-cathode fluorescent lamp cannot be achieved.
The present invention has as its object the provision of a cold-cathode fluorescent lamp which can solve the above-mentioned problem. The cold-cathode fluorescent lamp can suppress sputtering caused by electric discharge and reduce consumption of mercury, thereby achieving longer lifetime.
A cold-cathode fluorescent lamp of the present invention comprises a sealed lighting tube having a phosphor applied onto its inner surface, and a cylindrical electrode provided on an end of the lighting tube. The phosphor provided inside the lighting tube is excited by ultraviolet radiation generated inside the lighting tube by electric discharge to provide visible radiation. The cold-cathode fluorescent lamp is characterized in that a distance between the inner surface of the lighting tube and an outer surface of the cylindrical electrode is regulated such that the electric discharge develops mainly on the inner surface of the cylindrical electrode.
According to the present invention, even if a current is large and a diameter of the lighting tube is small, sputtering of the electrode can be suppressed and a consuming speed of mercury is suppressed so as to achieve a longer lifetime of the cold-cathode fluorescent lamp. A cold-cathode fluorescent lamp according to aspect 1 comprises a sealed lighting tube having a phosphor applied onto its inner surface, and a cylindrical electrode provided at an end portion of the lighting tube, the phosphor provided in the lighting tube being excited by ultraviolet radiation that is generated in the lighting tube by electric discharge to provide visible radiation. The cold-cathode fluorescent lamp is characterized in that a distance between the inner surface of the lighting tube and an outer surface of the cylindrical electrode is regulated such that the electric discharge develops mainly on the inner surface of the cylindrical electrode.
With this configuration, it is possible to suppress excessive sputtering and reduce a consuming speed of mercury, thereby achieving a longer lifetime of the cold-cathode fluorescent lamp. A cold-cathode fluorescent lamp according to aspect 2 of the present invention is characterized in that in aspect 1, the lighting tube has an inside diameter D1 of 1 to 6 mm, the cylindrical electrode has an outside diameter D2 of D1xe2x88x920.4 [mm]xe2x89xa6D2 less than D1, and a maximum lamp current is 5 mA or more.
With this configuration, an interval between the inner surface of the lighting tube and the outer surface of the cylindrical electrode can be sufficiently small to allow electric discharge to develop mainly on the inner surface of the cylindrical electrode. A cold-cathode fluorescent lamp according to aspect 3 of the present invention is characterized in that in aspect 2, the distance d between the inner surface of the lighting tube and the outer surface of the cylindrical electrode is 0 less than dxe2x89xa60.2 [mm].
With this configuration, even in an operating environment at a low temperature, in which the sputtering amount is larger than at a room temperature, no electric discharge moves to an interval between the inner surface of the lighting tube and the outer surface of the cylindrical electrode. Thus, it is possible to prevent sputtering from largely consuming mercury in a short period and to suppress shortening of lifetime caused by early consumption of the electrode. A cold-cathode fluorescent lamp according to aspect 4 of the present invention is characterized in that in aspect 1, the inner surface and outer surface of the cylindrical electrode are made of different materials, and the material forming the outer surface has a larger work function than that of the material forming the inner surface.
With this configuration, since electric discharge develops in the inside of the cylindrical electrode, the inside having a smaller work function, it is possible to synergistically suppress consumption of mercury that is caused by excessive sputtering, thereby achieving a longer lifetime of the cold-cathode fluorescent lamp. A cold-cathode fluorescent lamp according to aspect 5 of the present invention is characterized in that in aspect 1, an electronic emissive material is provided in the cylindrical electrode, the electronic emissive material containing a material having a smaller work function than that of a material forming the inner surface of the cylindrical electrode.
With this configuration as well, since electric discharge develops in the inside of the cylindrical electrode, the inside having a smaller work function, it is possible to synergistically suppress consumption of mercury that is caused by excessive sputtering, thereby achieving a longer lifetime of the cold-cathode fluorescent lamp.
A cold-cathode fluorescent lamp according to aspect 6 of the present invention is characterized in that in any one of aspects 1 to 4, the outer surface of the cylindrical electrode has protrusions abutting the inner surface of the lighting tube.
With this configuration, even when the cold-cathode fluorescent lamp has an ultra-small inside diameter of 1 to 6 mm, it is possible to prevent a contact between the cylindrical electrode and the inner wall of the lighting tube, thereby suppressing a local temperature increase on the outer wall of the lighting tube.